The present invention relates to electronic filter assemblies used in the cable television (CATV) industry, and more specifically to structural features of such filter assemblies directed to providing improved electrical grounding of a filter circuit carried on a printed circuit board.
In typical CATV applications a filter circuit or network is provided to pass signals having frequencies within one or more specified band widths, sometimes with a desired amount of signal attenuation, while blocking signals of other frequencies. It has been found convenient to mount the electrical components, e.g., capacitors, inductors and resistors, on one or more printed circuit boards in essentially conventional fashion. That is, for such CATV filters as highpass and lowpass filters, diplex filters, windowed highpass filters, and step attenuator (or return path) filters, tunable filter circuits and shielding between filter components and circuits are not normally required. The circuit board(s) carrying the filter circuit components is/are mounted within a suitable, protective housing. Prime requirements for the circuitry and housing means are preventing ingress of moisture and foreign matter through the filter connectors (such as shown in U.S. Pat. No. 5,278,525 to Palinkas) and the provision of a solid and reliable electrical ground between the filter circuit(s) and the housing (such as disclosed in U.S. Pat. No. 6,323,743 to Zelenz et al). The latter requirement is particularly critical when the filter is intended to handle signals of high frequency. In fact, the grounding arrangements of the prior art, including that of the Zelenz et al patent, are not fully reliable for frequencies above about 300 Mhz.
Filter circuits of this type are typically designed with a plurality of interconnected xe2x80x9csections,xe2x80x9d sometimes referred to as xe2x80x9cpoles,xe2x80x9d xe2x80x9cstages,xe2x80x9d or by other terms. As generally understood in the art, and as used herein, the term xe2x80x9csection,xe2x80x9d as applied to filter circuits, denotes a discrete portion of the circuit including a group of two or more components which establish a resonance (e.g., a capacitor and an inductor). In general, the performance of highpass, lowpass, and other types of CATV filters is enhanced by increasing the number of sections. That is, other factors being equal, a circuit having four sections is superior to a circuit having two sections.
Oftentimes, the greatest obstacle to achieving optimum circuit performance is proper grounding. Each section of the circuit requires a connection to ground. Traditionally, filter circuits carried in housings less than 2.1 cm in diameter make the ground contact between the housing and circuit sections at the male connector side of the housing. This is the case, for example, in the previously referenced Zelenz, et al patent where grounding pads are provided at each of the two corners of the board at the male connector end and on both surfaces of the circuit board. When the ground contact is placed at one end of the board, the electrical path from the circuit sections to the ground contact lengthens as the section components are placed farther away from the housing ground contact. Inductance and resistance parasitics are introduced into the electrical path in proportion to its length, i.e., the longer the path between the circuit section and the ground contact, the greater the level of undesirable inductance and resistance which are introduced. The additional inductance limits the circuit operating frequency range and the resistance severely degrades the xe2x80x9cQxe2x80x9d (quality factor) of the circuit. The present invention is based upon recognition of these effects and minimizing their impact on circuit performance.
The principal object of the present invention is to provide a filter assembly for CATV applications with improved electrical grounding of the filter circuit.
A further object is to provide a CATV filter assembly having a filter circuit with a plurality of sections each of which is connected to ground potential through an electrical path of minimal length.
Another object is to provide a CATV filter assembly including a circuit board carrying a filter circuit and mounted in a housing at ground potential wherein the board is mechanically connected and the circuit is electrically connected to the housing without a soldering operation.
A still further object is to provide a CATV filter assembly including a filter circuit carried on a printed circuit board mounted within a housing having two mating portions wherein the filter circuit is placed in electrical (grounding) communication with the housing by mutual assembly of the two portions.
Other objects will in part be obvious and will in part appear hereinafter.
The invention will be shown and described in three physical embodiments, although it will be understood that the electrical and structural components and their arrangement are subject to many modifications within the scope and spirit of the invention. In each disclosed embodiment the circuit components are mounted upon a single circuit board using surface mount components, although the invention may be implemented with filter circuits utilizing through-hole components, or a plurality of circuit boards. Likewise, the invention may be practiced with a wide variety of filter circuit designs although it is directed to use with circuits having a plurality of sections, each of which has a direct path to ground. That is, at least one component of each section of the circuit is connected by a path on the circuit board to a member which is at ground potential. The components are connected to one another and to the grounding member by electrically conducting paths on the board which are sometimes referred to as traces or runs, but in the present description and claims are uniformly called xe2x80x9cpaths.xe2x80x9d
The housing or casing wherein the circuit board is mounted is of electrically conducting material at ground potential, as in many conventional filter assemblies of this type, and provides the ultimate ground for the circuit. The paths from the respective sections may contact portions of the housing directly, or (as in the preferred embodiment) may extend from each section to a grounding bus on one or both surfaces of the board. The grounding bus is in direct electrical communication with the housing and thus provides a connection to ground for each section of the circuit components. The grounding bus may be (again, as in the preferred embodiment) in the form of an elongated strip, in which case it is important the strip be in direct contact with the housing over substantially all of its length so that an ungrounded portion of appreciable length will not act as a lengthy electrical path between the ground connection and one or more circuit sections, thereby contributing to the problem of inductance and resistance parasitics. In any case, the length of the electrical path from each section of the circuit to a member at ground potential, i.e., not simply to the grounding bus but to a point on the grounding bus which is in direct, grounding contact with the housing, is kept to a minimum which may be as small as one millimeter or less, but in no event is more than 5 millimeters.
In the first disclosed and preferred embodiment, the housing includes inner and outer, hollow, cylindrical portions. The inner portion includes a pair of open slots on opposite sides of the cylinder extending from one end for part of the axial length thereof. The printed circuit board carrying the filter circuit components and connecting paths has parallel, linear side edges with strips of conducting metal extending along marginal portions of the board adjacent each side edge on both the top and bottom surfaces of the board. The widths of the slots in the inner housing portion are greater than the thicknesses of the respective edge portions of the circuit board, and the outside diameter of the inner housing portion is slightly greater than the distance between the outer edges of the board.
In addition to the circuit board and components mounted thereon, and the two housing portions, the assembly includes male and female connector assemblies, each extending linearly between a first end, which is electrically and mechanically attached to the circuit board, and a terminal end. The assembly further includes an insulator and seal through which the female pin extends, an insulator through which the male connector extends and an O-ring forming a seal between the inner and outer housing portions. In assembly, the circuit board, with male and female connectors attached, is mounted to the inner portion by relative axial movement of the two parts with the edge portions of the board positioned between opposing edges of the slots on each side of the housing inner portion.
As previously indicated, the outer edges on each side of the board are positioned inwardly a small distance from the outer surface of the inner housing portion. The outer diameter of the inner portion is slightly greater than the inner diameter of the outer portion, at least over the parts of the two portions which are in mutually engaged relation in the fully assembled condition of the housing. Thus, as the inner and outer sections are moved axially into mating relation, there will be a tight, interference fit of the outer portion over the inner portion; in fact, it is preferred that the portions be assembled with the aid of an arbor press, or the like. The female and male connectors pass though openings in the insulators and seal with force fits. The O-ring is received in a groove in the inner portion for deformable engagement between the inner and outer housing portions.
As the outer portion is forced over the inner portion, the opposing edges of the slots on each side of the inner portion are moved toward one another, more markedly at the open ends of the slots. This urges the inner housing portion, at least in the region of the open ends of the slots, into tight contact with the grounding buses (strips) adjacent the lateral edges of the circuit board. As a further feature, one or more raised portions or xe2x80x9cbumpsxe2x80x9d of conductive material, e.g., soft solder, are provided on the surfaces of the grounding buses, at least in the regions thereof which are positioned near the closed ends of the slots. The edges of the slots are not narrowed as much near the closed ends as near the open ends by forced insertion of the outer over the inner portion. The bumps, in effect, increase the thickness of the circuit board in these regions, thereby ensuring a tight connection of the housing and grounding buses. An excellent electrical ground connection between each of the circuit sections and housing is provided by the force fit contact of the grounding buses of the circuit board with the inner housing portion over substantially all of the buses"" length while maintaining the minimal length of the electrical paths from the circuit sections to the buses. Likewise, excellent protection against penetration of moisture or foreign matter into the interior of the assembled housing is provided by the interference fit of the inner and outer housing sections, as well as the deformable seals and O-ring. Thus, in the preferred embodiment, the force fit of the housing portions and mechanical engagement of the board edges between opposing edges of the slots permits fully grounded assembly without the usual soldered connections.
In the second embodiment, the grounding buses along the side edges of the board are connected to the housing along their full length by soldering, while maintaining the minimal length of the electrical paths between all circuit sections and the grounding point. In a third embodiment, the location of the grounding bus(es) is/are at a position other than along marginal side edges of the board, while retaining the minimal path lengths and without the necessity of soldered connections. A version of the circuit board layout is disclosed wherein the paths from each section directly contact the housing rather than a grounding bus on the board.
The foregoing and other features of construction and operation of the filter assembly of the invention will be more readily understood and fully appreciated from the following detailed description, taken in conjunction with the accompanying drawings.